1. Field of the Invention
This invention relates to a phase change optical recording medium. More particularly, it relates to a phase change optical recording medium capable of high-speed direct overwriting.
2. Description of the Prior Art
As a practical example of an overwritable optical disc, employing a phase change recording material, a so-called DVD-RAM is commercialized, in which a linear velocity of 6 m/sec, a bit length of 0.41 .mu.m, track pitch of 0.74 .mu.m, a laser wavelength of approximately 650 nm, a data transfer rate of 11 Mbpa and a recording capacity of 2.6 GB have been realized.
For realizing a larger capacity and a higher transfer rate, it is effective to reduce the spot size of the recording laser and to raise the recording linear velocity. As a practical technique for reducing the spot size of the recording laser light, there are a method of shortening the laser wavelength and a method of increasing the numerical aperture of the objective lens.
In particular, if the method of reducing the laser wavelength and the method of increasing the numerical aperture of the objective lens are used in combination, the spot size may be made smaller than if one or the other method by itself is used. For example, if a blue to purple laser having a wavelength in the vicinity of 400 nm is used as a light source and an objective lens with a numerical aperture NA of 0.85 is used, it is theoretically possible to realize a still higher recording density.
However, the condition for performing direct overwrite at an elevated speed on the phase change optical disc is more stringent for the following reason:
In general, in a phase change optical disc, the laser light of a high power is illuminated thereon to fuse the recording layer by raising its temperature to higher than the fusing temperature of the recording layer. The recording layer then is quenched to write recording marks. The recorded marks are crystallized, that is erased, by being maintained in a temperature range intermediate between the crystallization initiating temperature and the melting point of the recording layer for a time necessary for the recording layer to become crystalline.
If the method for shortening the laser wavelength or the method of increasing the numerical aperture of the objective lens is adopted, and/or under the conditions of a higher recording linear velocity, the temperature on a point on the optical disc is changed within a shorter time than conventionally. FIG. 24 shows, as an example showing the results of how the temperature on a given point on the disc is varied with lapse of time. As may be seen from FIG. 24, the time during which the temperature is kept higher than the crystallization temperature, assumed to be 400.degree. C. as an example, becomes shorter the shorter is the recording and/or reproducing wavelength.
Thus, in a recording material having the crystallization speed which is the same as the conventional speed, it becomes difficult to crystallize, that is to erase, the recorded amorphous marks.
If, in case of a direct overwrite (DOW) in which recording is made as the sole laser light power level is controlled temporally, the spot size is small, or the recording linear speed is high, the result is promoted distortion of the mark shape ascribable to the difference in the physical properties between the amorphous phase and the crystalline phase, as characteristic of the phase-change recording. That is, under this condition, if a mark is overwritten in superposition on already written recording marks, the recording marks tend to be larger in size than if the marks are newly written in a blank crystalline state area.
This is caused by the difference in the response to the laser light (optical constant), the manner of heat transmission caused by the reaction with the laser light (thermal conductivity) or in the manner of use of the heat between the amorphous phase and the crystalline phase. It is noted that latent heat at the time of fusion is unnecessary. Meanwhile, if the spot size is large, with the linear speed being low, temporary temperature changes of the recording film are moderate, such that heat is conducted before the already present marks reach the laser so that the crystallization temperature is kept from the outset. Therefore, the state equivalent to a state in which the marks in actuality were not present (preceding crystallization) is realized, thus evading the aforementioned problem.
Due to this problem, a higher recording density and a higher transfer rate are difficult to achieve in case of a routinely used phase change optical disc having a fourlayer structure comprised of a ZnS--SiO.sub.2 layer, a recording layer, a ZnS--SiO.sub.2 layer and a reflective layer with the Ac/Aa ratio being lower than 0.9, Ac being a rate of absorption for the recording layer in the crystalline state and Aa being a rate of absorption for the recording layer in the amorphous state. For example, it has been shown experimentally that, as the linear speed is higher, the jitter value become worse, as shown for example in FIG. 25.
For solving this difficulty, it may be envisaged to increase the crystallization temperature of the recording layer. That is, it suffices if the time necessary for crystallization is shortened to raise the rate of erasure to render preceding crystallization liable to occur more readily.
However, it is virtually impossible to find a material in which phase change may occur reversibly and which has a crystallization temperature not lower than that realized in the past and to apply this material to an optical disc.
With this in view, there is proposed in Japanese Laying-Open Patent H-1-92937, Japanese Laying-Open Patent H-6-195747 or in Japanese Laying-Open Patent H-9-532424 a technique in which the crystallization promoting material, effective in improving the crystallization speed of the amorphous phase is provided adjacent to the recording layer in place of raising the crystallization speed of the recording material itself, in order to increase the crystallization speed of the recording layer. This technique has, however, a drawback that it tends to lower the storage stability of the recording marks in a temperature range encountered under the routine using conditions and that the recording marks are erased by the laser light used for reproduction.
On the other hand, there is also proposed in, for example, Japanese Laying-Open Patent H-8-124218 or Japanese Laying-Open Patent H-9-91755 a technique in which the state of the rate of absorption usually higher in the amorphous phase than in the crystallization phase is reversed by controlling the layering structure of the optical thin film, that is in which the temperature rising rate of the crystalline portion and that of the amorphous portion are balanced relative to each other to correct the distortion of the mark shape, that is the technique of controlling the rate of absorption to correct the distortion of the mark shape.
This technique has, however, a drawback that the degree of freedom in optical designing, such as rate of absorption or reflectance, is lowered, and that the durability against repeated overwriting is not that high.
Moreover, in the short wavelength conditions, the time during which the temperature higher than the crystallization temperature is maintained becomes shorter, with the result that the basic erasure capability, such as erasure ratio by the dc light, which is an indispensable condition for producing the effect of controlling the rate of absorption, falls short.